1. Technical Field
The present disclosure relates to a radar apparatus and a radar method.
2. Description of the Related Art
In recent years, a radar apparatus has been under consideration which uses short-wavelength radar transmission signals including microwaves or millimeter waves that yield high resolution. Further, for improvement in outdoor safety, there has been a demand for the development of a radar apparatus (wide-angle radar apparatus) that detects objects (targets) including pedestrians, as well as vehicles, in a wide angular range.
For example, as a radar apparatus, a pulse radar apparatus has been known which repeatedly emits pulse waves. A signal received by a wide-angle pulse radar that detects a vehicle or a pedestrian in a wide angular range is a mixture of a plurality of reflected waves from a target (e.g. a vehicle) that is present at a short distance and from a target (e.g. a pedestrian) that is present at a long distance. This requires (1) a radar transmitter to be configured to transmit a pulse wave or a pulse-modulated wave having an autocorrelation characteristic that forms a low-range side lobe (such a characteristic being hereinafter referred to as “low-range side lobe characteristic”) and requires (2) a radar receiver to be configured to have a wide reception dynamic range.
The following two configurations are possible as configurations of a wide-angle radar apparatus.
The first configuration employs transmitting radar waves by mechanically or electronically scanning narrow-angle directional beams (with beams width of approximately several degrees) of pulse waves or modulated waves, and receiving reflected waves by receiving narrow-angle directional beams. The first configuration requires much scanning for high resolution and, as such, is less capable of tracking a fast-moving target.
The second configuration employs a technique (direction of arrival (DOA) estimation) in which reflected waves are received by an array antenna including a plurality of antennas (antenna elements) and the angles of arrival of the reflected waves are estimated by a signal-processing algorithm based on a phase difference in reception due to intervals between antennas. The second configuration, which allows the angles of arrival to be estimated at a receiving branch in a case where scanning intervals between transmission beams at a transmitting branch are skipped, achieves a reduction in scanning time and, as such, is higher in tracking capability than the first configuration. Examples of direction-of-arrival estimation methods include a Fourier transform based on a matrix operation, a Capon method based on an inverse matrix operation, an LP (linear prediction) method based on an inverse matrix operation, MUSIC (Multiple Signal Classification) based on an eigenvalue operation, and ESPRIT (estimation of signal parameters via rotational invariance techniques) based on an eigenvalue operation.
Further, as a radar apparatus, a configuration (sometimes referred to as “MIMO radar”) has been proposed which includes a plurality of antennas (array antenna) at a transmitting branch as well as at a receiving branch and performs beam scanning by signal processing with the transmitting and receiving array antennas.
The MIMO radar multiplexes signals by frequency division or code division and transmits the multiplexed signals through the plurality of transmitting antennas. The MIMO radar receives, through the plurality of receiving antennas, signals reflected by surrounding objects (targets), demultiplexes the multiplexed transmitted signals from the respective received signals, and receives the demultiplexed signals. This allows the MIMO radar to take out a propagation channel response that is indicated by the product of the number of transmitting antennas and the number of receiving antennas. Further, the MIMO radar makes it possible to, by placing the transmitting and receiving antennas at appropriate intervals, virtually expand antenna openings and improve angular resolution.
For example, Japanese Unexamined Patent Application Publication No. 2008-304417 discloses a MIMO radar (hereinafter referred to as “time-division multiplexing MIMO radar”) based on time-division multiplexing by switching among transmitting antennas. The time-division multiplexing MIMO radar outputs transmit pulses while successively switching among the transmitting antennas, from which transmit pulses are sent out, with a predetermined period T. Moreover, the time-division multiplexing MIMO radar receives, through a plurality of receiving antennas, signals produced by transmit pulses being reflected by objects, performs correlation processing between the received signals and the transmit pulses, and then performs a spatial FFT (fast Fourier transform) operation (process of estimating the directions of arrival of the reflected waves).
However, the time-division multiplexing MIMO radar suffers from a phase difference due to the switching among the transmitting antennas. Therefore, prior to the spatial FFT operation, the time-division multiplexing MIMO radar needs to make phase corrections to the received signals in consideration of the timings of switching among the transmitting antennas.